
COMMENTS

The term a(5) = 99999999999999999999 is a fake, inserted to prevent this sequence being incorrectly returned by Superseeker. Of course it will be replaced by the correct value, once that is determined.
The present state of knowledge, quoting from Ge & Lam and the link below, is that a TWh(n) exists iff n == 0 or 1 (mod 4), except for n = 5, 9, 12 and possibly excepting n in {17, 57, 65, 69, 77, 85, 93, 117, 129, 153}.
After 16, the sequence continues 17?, 20, 21, 24, 25, 28, 29, 32, 33, 36, 37, 40, 41, 44, 45, 48, 49, 52, 53, 56, 57?, ...
